A. Rx Lens Market Trend to Polycarbonate
The relevant product field is vision-corrective plastic ophthalmic prescription spectacle lens (hereinafter abbreviated “Rx lens”) having refractive index greater than 1.530 glass and 1.49-1.50 “CR-39” (chemically, peroxide-crosslinked allyl diglycol carbonate thermoset-cast lens). This is the fastest growing category of Rx lens materials in the last five years, both in U.S. and worldwide markets. Such cast thermoset and injection-molded thermoplastics are so highly desirable because the consumer/wearer of spectacle lens finds them to be thinner (due to greater light-bending power of high-refractive-index plastic) and lighter (lower specific gravity, particularly in the case of polycarbonate versus CR-39). As a result, the myopic (“near-sighted”) spectacle lens wearer can avoid the cosmetically undesirable appearance of “wearing coke-bottle glasses”. In addition, lighter weight means better comfort, less weight, less pinching at the nose and top of ears, where the loadbearing surfaces are.
Within this “thin & light”, higher-refractive-index plastic Rx lens segment, U.S. market statistics show a combined share of 25-30% of the total market. However, within this segment, the thermoset cast high-index share has been essentially unchanged since 1991; nearly all this growth in recent years is of the thermoplastic injection-molded Rx lens type, most specifically embodied by polycarbonate (R.I.=1.586). (Although there are other candidate high-index thermoplastics also being considered, so far polycarbonate is most firmly established commercially—hereinafter, “polycarbonate” will be taken to be inclusive of other optical-grade thermoplastic substitutes, as would be obvious to those skilled in the art).
The major reason for market share shift toward polycarbonate Rx lens and away from cast thermoset high index Rx lens is reported to be the considerably lower manufacturing costs of polycarbonate Rx lens at high production volume levels. This, in turn, is from the high levels of automation attainable with polycarbonate, but inherently not attainable with the more labor-intensive thermoset casting operations. At low-volume percent utilization, highly automated production can be burdened with extremely high fixed cost, but as volume increases past “breakeven” levels, there is a cross-over point where the relatively higher variable-cost inputs of labor and materials inherent to thermoset casting becomes very disadvantageous. Thereafter, with increasing volume, the incremental profit per unit of increased volume becomes highly leveraged in favor of the more automated (polycarbonate) manufacturing operation.
This is reflected in market pricing from the lens manufacturers, wherein the cast high-index hardcoated Rx lenses are far from being price-competitive with corresponding prescriptions of the multi-cavity injection-molded, hardcoated polycarbonate Rx lenses (especially, finished single vision (“FSV”) types which have higher unit sales volumes per Rx). The cast high-index RSV can be typically 50-100% higher priced. It is for these reasons why a further level of manufacturing cost reduction, through even greater level of automation and through improved capital efficiency (=lower breakeven volume, which reduces capital requirements for new manufacturing entries into the field) will be strategically crucial in the polycarbonate Rx lens' future growth.
B. Prior Art Patents on Multi-Cavity Lens Molding and Dip Hardcoating
Today, polycarbonate Rx lens worldwide production is dominated by four companies, together comprising an estimated greater-than-90% share of world market (although there are new entries just starting up). Each of these four currently employ some form of injection-compression multi-cavity molding process and apparatus, at the start of their “batch process” manufacturing flowsheet (see FIG. 4A Comparative Example). The next step is post-molding cutting of runner system and/or degating or trimming off ejector tabs, so the trimmed lenses can be mounted into a lensholder rack. Typically, these are semi-automatic operations assisted by a human operator, but they can also be entirely manual operations. An example of a molded-on hanger tab which is fitted to engage a Lensholder rack holding a plurality of such lenses is shown in Weber (U.S. Pat. No. 4,443,159). The next step in the manufacturing flowsheet is to use some form of cleaning protocol (earlier versions were all Freon (tm) CFC ultrasonic vapor degreaser methodologies; more recently, water-based cleaning is aqueous high-pressure sprays with centrifugal spinning, or multi-stage ultrasonic tank immersions, followed by drying operations). These cleaned and dried lenses are then dipcoated in liquid hardcoating solutions (either heat-curing silicone types or UV-curing types), and the coating is cured by chemical crosslinking.
Two of the above-mentioned four polycarbonate Rx lens manufacturers are licensees of Applicants' U.S. Pat. No. 4,828,769 and U.S. Pat. No. 4,900,242. A third is Gentex Corporation, assignee of Weymouth (U.S. Pat. No. 4,933,119). A fourth is Neolens, assignee of Bakalar (U.S. Pat. No. 4,664,854). These patents employ some form of injection-compression molding process sequence with a plurality of mold cavities and employing various means for achieving cavity-to-cavity balance therebetween. These three patents employed by four manufacturers differ in how the molded lens is ejected out of the lens mold, as can be easily seen by observing the O.D.-perimeter lens edge & sidewall of a sample lens from each manufacturer. More on this later in FIG. 2 and its descriptive text. All three necessarily do at least some cutting before dipcoating is possible.
Looking at other prior art patents showing multicavity injection-compression molding of Rx lens, Weber (U.S. Pat. No. 4,008,031) apparatus for injection-compression molding of Rx lens shows what appears to be a two-cavity mold. At 180 degrees opposite the gate inlet 23 is a hanger 20 for use in subsequent dipcoating operations. Weber also shows two molded-on ejector tabs 16, located at about 10:30-1:00 o'clock positions, with respect to the gate/dripmark location at 6:00 o'clock. Normally, this location would have the detrimental effect of propagating coating flowout runs along the front and back faces of the molded lens during dipcoating withdrawal, but in Weber's case, he has installed the hanger tab and ejector tabs onto a circumferential flange 12, which is set back from both the front and back lens edges, such that coating flow runoff could then follow this flange from top to bottom of each individually-held lens (provided the lens don't swing from side to side).
Uehara et al (U.S. Pat. No. 5,093,049) also teaches and shows injection-compression molding of Rx lens in a two-cavity mold, with the cavities connected by a cold runner and sprue, with the sprue being able to be mechanically shut off at a predetermined time in the cycle, to prevent backflow. Uehara is silent on any ejection means for demolding these two lenses and no ejector tabs or pins are shown. If the forward travel of the movable cores, which provide the compression, is limited by hard stops, they cannot be used to drive forward past the parting line once the mold is open, to assist ejection. In that case, a human operator would be relied upon to manually grasp the cold sprue and pull loose the two lenses attached thereto from the mold. No hanger tab is shown or mentioned.
Other historically important injection-compression molding of Rx lenses includes Spector et al (U.S. Pat. No. 4,836,960) and Laliberte (U.S. Pat. No. 4,364,878), but both of these are limited to single-cavity embodiments.
Looking now at Rx lens dipcoating prior art patents (in additioon to previously-cited Weber (U.S. Pat. No. 4,443,159), Laliberte (U.S. Pat. No. 3,956,540 Method and U.S. Pat. No. 4,036,168 Apparatus) teaches a form of conveyorized transfer of such lensholder racks through a multistation machine internally having a filtered-air cleanroom environment, wherein the lenses are successively ultrasonically cleaned and destatisized, then dipcoated, then dried and at least partially cured to a tackfree state before the conveyor takes them to a loading/unloading station, where the lenses can be removed by the operator. Similar configurations were developed using different automated transfer means, including two chain-drive conveyors operating in parallel and connected by crossbars whereon the lensholder racks would be hung, or, alternatively, an overhead conveyor with power and free flights for indexing could be used, with suspended removable lensholder racks mounted thereon. Such configurations for polycarbonate Rx lenses (and non-Rx lenses) typically used at least one (preferably, two, in series dips) Freon ultrasonic cleaner/degreasers, wherein the polycarbonate lenses were immersed in the ultrasonic sump for a prescribed time, during which cavitation (generation and collapse of microscopic bubbles) provides high kinetic energy working synergistically with the Freon's solvency (to reduce adherent films holding onto the soils on the lens surface), to thus dislodge and float away surface contaminants of both soluable and insoluable types. After lens removal from the ultrasonic sump solution, an azeotropic freon/alcohol vapor zone would help rinse and dry the lens before going into the dipcoating tank.
Liebler et al, UK Patent Application GB2 159 441 A, published Dec. 4, 1985; assignee: Rohm GmbH) also teaches continuous dip production of scratch-resistant liquid coatings onto plastic optical moldings (such as lenses). It specifically teaches an endless conveyor belt to transfer lensholder racks-containing a plurality of lenses. Among the optical plastic moldings contemplated are spectacle lenses, and FIG. 2 shows a molding with a “lug 10 for clamping purposes is formed thereon and diametrically opposite this lugged end is a dripoff lug 11, so that excessive scratch-resistant coating composition can drip off without forming a ridge when coated and dried.” (Lines 97-105). In comparison to Laliberte, this machine is far simpler, contemplating merely a load/unload, a liquid dipcoating station, and a drying station shown (described as, “preferably, two or more infrared radiators”. Not shown but mentioned in text is . . . “cleansing bath may also be provided upstream of the immersion bath. The cleansing bath may, for example, be an ultrasonic bath containing organic solvent”. (Lines 122-128). However, Liebler is believed not to have ever been actually used for spectacle lens coating nor Rx lens coating. There are major technical problems unforeseen by Liebler. His FIG. 2 lens with diametrically-opposed hanger tab and drip tab would inevitably have coating flowout runs propagated from the two junctions of the coating tab, at its shoulders. Unfortunately, these runs take place in the very worst location of the perimeter, since the coating flow runs will go directly through the central, most critical zone of the optics for vision (see Comparative Example FIG. 2D). To the extent that the Liebler apparatus might be acceptable, it would not be believed to be spectacle lenses, but rather ordinary protective-covering lenses such as watch glasses, scales, and mirrors, none of which are required to have the high quality of image transmission that corrective-vision spectacle lenses must have. Where the hardcoating merely is to protect from heavy scratching and the protective-covering lens is merely to provide some transparency to a product or device, such flow runs may be harmless and not a functional problem. However, for spectacle lenses with human vision problems resulting from optical aberrations, such coating flow runs would be completely unacceptable and the source of very high percent rejectable flaws. If such tab configurations are as shown, of the full thickness of the lens molding, then such a problem would be absolutely intrinsic. However, if the tab is not of the full thickness of the lens, as shown in the Weber drawings, but merely thick enough to support the relatively light weight of the lens suspended thereby, then such a tab location would be acceptable, but only if the lens is held level in its mount, not rocking back,;and forth, which would be a another problem envisioned with Liebler's “endless conveyor”.
C. Environmental and Economic Problems with Lens Cleaning
“Freon” cleaning is based upon now-unacceptable CFC-113 (ozone-depleting), production of which theoretically ceased on Dec. 31st, 1994, in accordance with the Montreal protocol and its Eu revisions. As a result, new Rx lens installations necessarily have substituted aqueous cleaning approaches instead. One such approach employs high-pressure (up to 20,000 psi) jets of water spray which are scanned across the front and back surfaces of the lens, by moving the lens (such as spinning it on a spindle) or by moving the spray head (such as by reciprocating motion) or preferably, a combination of both. High-pressure water spray is very effective in removing insoluble particulate forms of surface contamination (such as electrostatically-held polycarbonate dust particles or airborne inorganic dusts) but has the drawback that such cleaning is 100% “line of sight”, so not only must lenses typically be cleaned one at a time, but a typical spin/spray combination requires one side to be cleaned, then manually or robotically flipped over and placed back on a different spindle to clean the second side. The throughput of such equipment (number of lenses per hour) versus the labor cost and capital cost is very much higher than the old Freon cleaners it replaced, which are now environmentally unacceptable.
A second way of aqueous cleaning is to have an ultrasonic, water-based detergent solution in the first stage of a countercurrent-flow, multi-station, automated cleaning line with conveyorized transport taking the lenses through successive immersion tanks (typically, at least five, and preferably 7-15 stations, including deionized water rinses).
Whether by high-pressure water spray or by ultrasonic, multi-stage tank immersions, the resulting clean-but-still-wet polycarbonate lens cannot yet be dipped into the liquid hardcoatings (which are all chemically incompatible with any significant % water), so they still face another problem, and that is how to completely remove all the remaining water from the lens (and/or its lensholder rack), without creating superficial stains (“water spots”) on the lens' optical surfaces. In the case of water-immersion tanks, the last tank is typically maintained at a very high temperature, near the boiling point of water (which can cause lens “fogging” due to high % humidity inside the cleanroom wherein dipcoating drydown must also be done), and the withdrawal rate of the lenses being removed from the tank is extremely slow, to encourage capillary effect to maximize water removal. In the case of spin/high-pressure spray, (centrifugal action of high-RPM spinning speeds is attempted to sling off all excess water. Nevertheless, because the liquid hardcoating solutions cannot stand even small amounts of water “dragout” introduced by lenses (even small droplets of water will result in streaky or spotty fogging of the coated lenses or blotchy appearance). So, inevitably, a hot-air-circulating dryer (filtered for cleanliness) must be used, which makes for an energy-intensive and costly operation. The multi-station automatic-transfer water cleaner in-line system takes up a great deal of floor space and costly (multi—$100,000). In addition, disposal of the liquid effluent from these aqueous cleaning solutions is turning out to be an environmental problem not previously encountered with the Freon cleaners it replaced.